It is important to be able to detect radioactive radiations on a nuclear site, in particular before dismantling thereof, in order to know the risks that the persons present on the site are incurring and to provide them with suitable protection.
Certain radiations are easier to detect than others. For these radiations, such as the γ (gamma) and X types, suitable tools are available today for knowing the intensity of these radiations in a localised manner.
However, other radiations are more difficult to detect since these are too weak, such as those of the β− type. β− radiations are emitted by atom nuclei, comprising neutrons and protons, presenting an excess of neutrons. At constant pressure, a neutron of the nucleus may transform into a proton. This is accompanied by the emission of an electron, which is a β− particle, and an antineutrino. β− radiations are termed moderately penetrating; a sheet of aluminium a few millimetres thick suffices to stop β− radiations, unlike γ and X radiations, which are termed highly penetrating.
In nuclear sites, β− radiations are due mainly to tritium 3H, a radioactive isotope of hydrogen 1H. Before proceeding with the dismantling of the site, it is wished to identify the areas emitting β− radiations, which reveals the presence of tritium 3H that it is necessary to decontaminate.
At the present time, in order to detect β− radiations due to tritium 3H on a solid surface, samplings are carried out, which destroys the solid surface being studied.
For example, in the document “Calculations and measurements of the activation induced in the protective concretes of a high-energy ion accelerator”, in Radioprotection 2000, volume 35, n° 3, pages 311 to 334, the dismantling of the Saturn II synchrotron is described, in operation from 1979 to 1997 at Saclay, France. Prior to the measurement, core sampling is carried out in the concrete of the synchrotron structure, and then the core taken is sampled with diamond. The tritium 3H is then measured on these cores by counting with a liquid scintillator. The sample preparation method is not known from this document.
One drawback of the method used in this article is the need to destroy the solid surface on which it is wished to carry out the measurements of tritium 3H. Another drawback is the very localised measurement of tritium 3H and, if a measurement at another place is required, another core sampling is necessary.
A device and method for measuring tritium 3H in a non-destructive manner are described in the document JP 3-041 386. The device comprises a connection head composed of one end of an aspiration tube, heating elements and fixing suckers for fixing to a solid surface. The aspiration tube is connected to a cold trap, itself connected to an aspiration pump provided with a flow meter.
The method used is as follows: the connection head is placed on the solid surface, in this case concrete. The fixing suckers hold the connection head in place. The heating elements heat the concrete 200° C. on the part covered by the connection head, thus evaporating the water present in the concrete. The pump sucks the water vapour escaping from the concrete. When the water vapour reaches the cold trap, it cools until it condenses into liquid water, which will be used subsequently for measuring the tritium 3H contained in this water.
One drawback of the device and method of this document is that discretising the solid surface is only possible in fractions of the size of the connection head. However, the size of the connection head must be sufficiently large to make it possible to aspirate a sufficient quantity of water in vapour form especially since the concrete does not contain an enormous amount thereof. This technique also assumes that the tritium 3H is labile whereas it may very well be strongly bonded to the concrete.
Another drawback is the need to transport potentially radioactive liquid samples.
Yet another drawback is the difficulty of robotising the placing of the connection head, which has a specific form.